1. Field of the Invention
The present invention relates to a nuclear magnetic resonance exciting device such as a magnetic resonance imaging device, and in particular to a nuclear magnetic resonance exciting device having means compensating magnetic field produced by eddy current due to a gradient magnetic field.
2. Description of the Related Art
In a magnetic resonance imaging (MRI) device, which is a kind of nuclear magnetic resonance exciting devices, since magnetic field produced by eddy current due to a gradient magnetic field disturbs the gradient magnetic field, usually a measure for compensating it is taken.
Such a technique for compensating the magnetic field produced by eddy current is disclosed in U.S. Pat. No. 4,585,995 published Apr. 29, 1986. Analyzing the electric circuit part for compensating eddy current disclosed therein, it can be known that the transmission function used for the compensation in the eddy current compensating circuit is given by following Eq. (2) obtained by adding the total sum of transmission functions expressed by Eq. (1) and 1, part corresponding to the input; ##EQU1## where k.sub.i, T.sub.i .gtoreq.0 ##EQU2##
As it can be clearly seen from above Eq. (2), in the construction for compensating the eddy current a method is adopted, by which influences produced by the eddy current are preestimated and fed back in order to generate an additive approximative inverse function, and neither circuit theoretical consideration on the cause of generation of the eddy current nor circuit theoretical construction of the eddy current compensating method is effected.
Originally, in the case where waveform distorsion in the gradient magnetic field due to eddy current is corrected in the eddy current compensating device for an MRI device, perfect compensation cannot be effected, unless such a transmission function that it compensates a transmission function including a factor of generating waveform distorsion is disposed in the compensating circuit portion. In the eddy current compensating circuit described above no attention is paid at all with respect to this point.
Further the circuit portion for the eddy current compensation described above has a problem that adjustment precision cannot be high, because it is constructed by adopting the additive approximative inverse function generating method and therefore adjustment is troublesome because of a number of parameters included for the adjustment and at the same time errors are accumulated for different parameters at the adjustment because of the construction.